CIRCULATORY SYSTEM 797 



the exception of a posterior portion which takes part in the forming 

 of the post cava, and the right anterior portion which connects with the 

 remnant of the post cardinal to become the azygous vein. If the an- 

 terior left side persists also, this is known as the hemiazygous. 



In the lower vertebrates they retain their function of draining the 

 excretory system. 



Cuvierian Ducts: 



These are formed by the meeting of the anterior cardinal and the 

 post-cardinal vein on each side to form short tubes for the emptying 

 of the cardinal veins into the sinus venosus. 



Subcardinal Veins: 



These are closely associated with the post-cardinals. 



As the mesonephroi in their development reach the hinder end of 

 the coelom, the caudal vein loses its primitive connection with the sub- 

 intestinal vein and becomes connected with a pair of vessels, the sub- 

 cardinal veins, which develop in a ventral-medial position to the two 

 mesonephroi. The blood from the tail now goes through the subcardi- 

 nals and from them into the excretory organs, passing through a system 

 of capillaries to be gathered again in the postcardinals and by them to 

 be returned to the heart. Here, then, there is another portal system, the 

 first renal-portal system, which may be modified later. 



Subclavian Veins: 



One of these drains each forelimb. It originally empties into the 

 post-cardinal but later may empty into the Cuvierian duct or jugular 

 vein. 



Common Iliac Vein: 



This drains the hind limb and empties into the epigastric (lateral 

 abdominal) vein, which in turn empties into the post-cardinal or duct of 

 Cuvier. 



While this is the condition in some elasmobranchs, in the reptiles 

 and amphibia the common iliac sends part of its blood as above, and part 

 through the post-cardinal of its ow-n side, so that blood from the hind 

 limbs has two routes to the heart. 



Anterior Abdominal Vein: 



In amphibia and some reptiles the two epigastric veins fuse in the 

 midline to form an anterior abdominal vein, which passes through the 

 remains of the ventral mesentery (ligamentum teres), to the liver and 

 forward. 



In one mammal, Echidna alone, has such an anterior abdominal vein 

 been found. 



The vessels of the appendages are but slightly developed in fishes. 

 There is a subclavian vein which enters the Cuvierian duct, and some- 



